The 3GPP Long Term Evolution (“LTE”) Specifications define two types of interference mitigation techniques: The first one being interference mitigation by interference reduction, and the second one is interference mitigation by inter cell interference coordination (ICIC). The 3GPP standard handles the two types of interference minimization differently. The first type, interference reduction, is used in conjunction with coverage and capacity optimization. The enablement of interference reduction are RF techniques such as antenna tilt, transmit power reduction, and handover mechanisms.
The LTE Recommendation has defined an interface between base stations (referred to herein as “eNBs”) which enables the transfer of ICIC related indicators. This interface is referred to as X2. These ICIC function indicators are: Relative Narrowband Transmit Power Indicator (“RNTPI”), High Interference Indicator (“HII”), and Interference Overload Indicator (“OI”).
The RNTPI indicator message is sent to neighbor eNBs. It contains 1 bit per each physical resource block (PRB) in the downlink transmission, indicating if the transmission power associated with that PRB will be greater than a pre-defined threshold. Thus, neighbor eNBs may anticipate which bands would suffer more severe interference and take the appropriate scheduling decisions immediately, rather than waiting to and relying on the UEs' Channel Quality Information (“CQI”) reports.
The HII indicator for uplink transmissions has a somewhat similar function as that which was described above in connection with the RNTPI message for downlink transmissions. There is one bit per each PRB, enabling the neighboring eNBs to assess whether they should expect high interference power in the near future. Reference Signal Received Power (“RSRP”) measurements which are reported as part of handover measurement reports, can identify cell edge UEs. In a similar way, this indicator can be used to identify the bands used in a frequency-partitioning scheme.
While the previously described X2 messages are sent out proactively by eNBs, the OI indicator is only triggered when high-interference in the uplink direction is detected by an eNB. An overload indication will be sent to neighbor eNBs whose UEs are potentially the source of this high interference. The message contains a low, medium, or high interference level indication per each PRB. However, the question, which cell is the one responsible for the high interference is of course not a trivial question to answer.
According to the 3GPP Specifications, X2 based ICIC does not include any provisioning for a decision making process, consequently, ICIC algorithms in base stations, which are originated by different vendors, may use completely different logics and criteria. This essentially limits the X2-based ICIC solution to areas where the base stations originate from a single vendor. While in existing macro deployments this constrain might still be achieved, for modern multi-RAT networks (LTE overlay over UMTS network) and HetNet networks, such a requirement of having one vendor's equipment is too restrictive, if not impossible.
According to 3GPP TS 36.300, Inter-cell interference coordination is associated with managing radio resources (notably the radio resource blocks) such that inter-cell interference is kept under control. ICIC is inherently a multi-cell, radio resource management (“RRM”) function that needs to take into account information (e.g. the resource usage status and traffic load situation) obtained from various cells. Furthermore, an ICIC method may be different in the uplink and downlink.
The 3GPP Release 10 introduced a new LTE network concept for the heterogeneous networks (HetNets), in contrast to previous network releases, which deal with homogeneous networks. HetNet is defined in that release as a network of eNBs with different capabilities, most importantly, different Tx-power classes.
However, heterogeneous networks pose new ICIC challenges. A first ICIC challenge involves macro UE that roams about a Home eNB (HeNB) and is not part of the closed subscriber group (“CSG”). In that scenario the Macro eNB UE transmission will become uplink interference to the Home eNB authorized UEs. The second ICIC challenge is macro eNB transmission that forms downlink interference to Pico eNB center cell UE. In order to enable the use of HetNet, enhanced ICIC (eICIC) Rel. 10 requires that all members of a HetNet (Macro, Pico, HeNB) should be capable of interconnecting by using the X2 interface.
Soft Fractional Frequency Reuse (“FRR”) technique implements separation of transmissions in neighbor LTE cells. The separation is performed by allocating time-frequency resources in blocks (partitions) that appear as rectangles in time-frequency plane as illustrated for example in FIG. 1. This concept may be demonstrated in the following example. For uplink (“UL”) transmissions, a time-frequency block X can be allocated to a group of cell edge UEs in cell A, whereas time-frequency block Y is allocated to a group of cell edge UEs in cell B. If X and Y do not overlap, this scheme may alleviate mutual interference between cells A and B.
There are many FFR schemes used for HetNets that allow significant reuse of spectrum. For example in FIGS. 2A and 2B, where A . . . D denote certain “chunks” of the UL frequency channel that are allocated for use by Macro eNBs and for use by Metro eNBs, with differentiation between cell center and cell edge UEs. Such schemes reduce interference because adjacent cells (particularly a Metro cell that can be located within a Macro cell) are using separate orthogonal parts of the spectrum.
However, a solution is still required that enables automatic bandwidth partitioning for example between macro and metro eNBs, and particularly in cases where base stations produced by different vendors and located at the same geographical vicinity, are involved.